This project proposes to study the cellular mechanisms responsible for sex differences in susceptibility to infection and immunologic responsiveness. The functioning of cells of reticuloendothelial and lymphoid systems of male and female mice will be compared. The uptake of bacterial antigen by reticuloendothelial tissue and the relative ability of male and female macrophages after uptake of antigen to induce formation of antibody-producing cells will be studied. The relative effectiveness of the thymus of each sex to enhance humoral antibody production will be determined as will DNA production by male and female antibody-producing cells. These investigations should provide insight into the reasons why the male is more susceptible to infection and the female is more prone to disease attributed to abnormal or increased responsiveness of the immune system.